Avoid duplicated compuations with DateTime.GetDatePart

- Remove duplicate code in GregorianCalendar. GetDatePart is identical to the implementation in DateTime.  Just reuse it via the Year/Month/Day properties. Also avoids an unnecessary virtual call.
- Then copy GetDatePart into a version that returns year/month/day rather than just one of them.
- And use that in several places throughout corelib where all three are retrieved, avoiding redoing all of the calculations three times.

6 files changed, 63 insertions, 80 deletions

diff --git a/src/mscorlib/shared/System/DateTime.cs b/src/mscorlib/shared/System/DateTime.cs
index 4fd9727fcd..16a75fdfa6 100644
--- a/src/mscorlib/shared/System/DateTime.cs
+++ b/src/mscorlib/shared/System/DateTime.cs
@@ -453,9 +453,7 @@ namespace System
         {
             if (months < -120000 || months > 120000) throw new ArgumentOutOfRangeException(nameof(months), SR.ArgumentOutOfRange_DateTimeBadMonths);
             Contract.EndContractBlock();
-            int y = GetDatePart(DatePartYear);
-            int m = GetDatePart(DatePartMonth);
-            int d = GetDatePart(DatePartDay);
+            GetDatePart(out int y, out int m, out int d);
             int i = m - 1 + months;
             if (i >= 0)
             {
@@ -859,6 +857,51 @@ namespace System
             return n - days[m - 1] + 1;
         }
 
+        // Exactly the same as GetDatePart(int part), except computing all of
+        // year/month/day rather than just one of them.  Used when all three
+        // are needed rather than redoing the computations for each.
+        internal void GetDatePart(out int year, out int month, out int day)
+        {
+            Int64 ticks = InternalTicks;
+            // n = number of days since 1/1/0001
+            int n = (int)(ticks / TicksPerDay);
+            // y400 = number of whole 400-year periods since 1/1/0001
+            int y400 = n / DaysPer400Years;
+            // n = day number within 400-year period
+            n -= y400 * DaysPer400Years;
+            // y100 = number of whole 100-year periods within 400-year period
+            int y100 = n / DaysPer100Years;
+            // Last 100-year period has an extra day, so decrement result if 4
+            if (y100 == 4) y100 = 3;
+            // n = day number within 100-year period
+            n -= y100 * DaysPer100Years;
+            // y4 = number of whole 4-year periods within 100-year period
+            int y4 = n / DaysPer4Years;
+            // n = day number within 4-year period
+            n -= y4 * DaysPer4Years;
+            // y1 = number of whole years within 4-year period
+            int y1 = n / DaysPerYear;
+            // Last year has an extra day, so decrement result if 4
+            if (y1 == 4) y1 = 3;
+            // compute year
+            year = y400 * 400 + y100 * 100 + y4 * 4 + y1 + 1;
+            // n = day number within year
+            n -= y1 * DaysPerYear;
+            // dayOfYear = n + 1;
+            // Leap year calculation looks different from IsLeapYear since y1, y4,
+            // and y100 are relative to year 1, not year 0
+            bool leapYear = y1 == 3 && (y4 != 24 || y100 == 3);
+            int[] days = leapYear ? s_daysToMonth366 : s_daysToMonth365;
+            // All months have less than 32 days, so n >> 5 is a good conservative
+            // estimate for the month
+            int m = (n >> 5) + 1;
+            // m = 1-based month number
+            while (n >= days[m]) m++;
+            // compute month and day
+            month = m;
+            day = n - days[m - 1] + 1;
+        }
+
         // Returns the day-of-month part of this DateTime. The returned
         // value is an integer between 1 and 31.
         //
diff --git a/src/mscorlib/shared/System/Globalization/DateTimeFormat.cs b/src/mscorlib/shared/System/Globalization/DateTimeFormat.cs
index 840409f55a..d6afc1b099 100644
--- a/src/mscorlib/shared/System/Globalization/DateTimeFormat.cs
+++ b/src/mscorlib/shared/System/Globalization/DateTimeFormat.cs
@@ -1059,14 +1059,15 @@ namespace System
                 dateTime = dateTime - offset;
             }
 
+            dateTime.GetDatePart(out int year, out int month, out int day);
             result.Append(InvariantAbbreviatedDayNames[(int)dateTime.DayOfWeek]);
             result.Append(',');
             result.Append(' ');
-            AppendNumber(result, dateTime.Day, 2);
+            AppendNumber(result, day, 2);
             result.Append(' ');
-            result.Append(InvariantAbbreviatedMonthNames[dateTime.Month - 1]);
+            result.Append(InvariantAbbreviatedMonthNames[month - 1]);
             result.Append(' ');
-            AppendNumber(result, dateTime.Year, 4);
+            AppendNumber(result, year, 4);
             result.Append(' ');
             AppendHHmmssTimeOfDay(result, dateTime);
             result.Append(' ');
@@ -1081,11 +1082,12 @@ namespace System
             const int roundTripFormatLength = 28;
             StringBuilder result = StringBuilderCache.Acquire(roundTripFormatLength);
 
-            AppendNumber(result, dateTime.Year, 4);
+            dateTime.GetDatePart(out int year, out int month, out int day);
+            AppendNumber(result, year, 4);
             result.Append('-');
-            AppendNumber(result, dateTime.Month, 2);
+            AppendNumber(result, month, 2);
             result.Append('-');
-            AppendNumber(result, dateTime.Day, 2);
+            AppendNumber(result, day, 2);
             result.Append('T');
             AppendHHmmssTimeOfDay(result, dateTime);
             result.Append('.');
diff --git a/src/mscorlib/shared/System/Globalization/HebrewCalendar.cs b/src/mscorlib/shared/System/Globalization/HebrewCalendar.cs
index 6ba4f082f1..92f4cab4be 100644
--- a/src/mscorlib/shared/System/Globalization/HebrewCalendar.cs
+++ b/src/mscorlib/shared/System/Globalization/HebrewCalendar.cs
@@ -536,9 +536,7 @@ namespace System.Globalization
             //
             //  Save the Gregorian date values.
             //
-            gregorianYear = time.Year;
-            gregorianMonth = time.Month;
-            gregorianDay = time.Day;
+            time.GetDatePart(out gregorianYear, out gregorianMonth, out gregorianDay);
 
             __DateBuffer lunarDate = new __DateBuffer();    // lunar month and day for Jan 1
 
diff --git a/src/mscorlib/shared/System/Globalization/UmAlQuraCalendar.cs b/src/mscorlib/shared/System/Globalization/UmAlQuraCalendar.cs
index c03ac23d95..99b99d67c0 100644
--- a/src/mscorlib/shared/System/Globalization/UmAlQuraCalendar.cs
+++ b/src/mscorlib/shared/System/Globalization/UmAlQuraCalendar.cs
@@ -331,9 +331,7 @@ namespace System.Globalization
             }
 
             dt = dt.AddDays(nDays);
-            yg = dt.Year;
-            mg = dt.Month;
-            dg = dt.Day;
+            dt.GetDatePart(out yg, out mg, out dg);
         }
 
         /*=================================GetAbsoluteDateUmAlQura==========================
diff --git a/src/mscorlib/src/System/Globalization/GregorianCalendar.cs b/src/mscorlib/src/System/Globalization/GregorianCalendar.cs
index 9115b12bd0..707a9d01b1 100644
--- a/src/mscorlib/src/System/Globalization/GregorianCalendar.cs
+++ b/src/mscorlib/src/System/Globalization/GregorianCalendar.cs
@@ -21,12 +21,6 @@ namespace System.Globalization
 
         public const int ADEra = 1;
 
-
-        internal const int DatePartYear = 0;
-        internal const int DatePartDayOfYear = 1;
-        internal const int DatePartMonth = 2;
-        internal const int DatePartDay = 3;
-
         //
         // This is the max Gregorian year can be represented by DateTime class.  The limitation
         // is derived from DateTime class.
@@ -161,57 +155,6 @@ namespace System.Globalization
         }
 
 
-        // Returns a given date part of this DateTime. This method is used
-        // to compute the year, day-of-year, month, or day part.
-        internal virtual int GetDatePart(long ticks, int part)
-        {
-            // n = number of days since 1/1/0001
-            int n = (int)(ticks / TicksPerDay);
-            // y400 = number of whole 400-year periods since 1/1/0001
-            int y400 = n / DaysPer400Years;
-            // n = day number within 400-year period
-            n -= y400 * DaysPer400Years;
-            // y100 = number of whole 100-year periods within 400-year period
-            int y100 = n / DaysPer100Years;
-            // Last 100-year period has an extra day, so decrement result if 4
-            if (y100 == 4) y100 = 3;
-            // n = day number within 100-year period
-            n -= y100 * DaysPer100Years;
-            // y4 = number of whole 4-year periods within 100-year period
-            int y4 = n / DaysPer4Years;
-            // n = day number within 4-year period
-            n -= y4 * DaysPer4Years;
-            // y1 = number of whole years within 4-year period
-            int y1 = n / DaysPerYear;
-            // Last year has an extra day, so decrement result if 4
-            if (y1 == 4) y1 = 3;
-            // If year was requested, compute and return it
-            if (part == DatePartYear)
-            {
-                return (y400 * 400 + y100 * 100 + y4 * 4 + y1 + 1);
-            }
-            // n = day number within year
-            n -= y1 * DaysPerYear;
-            // If day-of-year was requested, return it
-            if (part == DatePartDayOfYear)
-            {
-                return (n + 1);
-            }
-            // Leap year calculation looks different from IsLeapYear since y1, y4,
-            // and y100 are relative to year 1, not year 0
-            bool leapYear = (y1 == 3 && (y4 != 24 || y100 == 3));
-            int[] days = leapYear ? DaysToMonth366 : DaysToMonth365;
-            // All months have less than 32 days, so n >> 5 is a good conservative
-            // estimate for the month
-            int m = (n >> 5) + 1;
-            // m = 1-based month number
-            while (n >= days[m]) m++;
-            // If month was requested, return it
-            if (part == DatePartMonth) return (m);
-            // Return 1-based day-of-month
-            return (n - days[m - 1] + 1);
-        }
-
         /*=================================GetAbsoluteDate==========================
         **Action: Gets the absolute date for the given Gregorian date.  The absolute date means
         **       the number of days from January 1st, 1 A.D.
@@ -283,9 +226,7 @@ namespace System.Globalization
                                 120000));
             }
             Contract.EndContractBlock();
-            int y = GetDatePart(time.Ticks, DatePartYear);
-            int m = GetDatePart(time.Ticks, DatePartMonth);
-            int d = GetDatePart(time.Ticks, DatePartDay);
+            time.GetDatePart(out int y, out int m, out int d);
             int i = m - 1 + months;
             if (i >= 0)
             {
@@ -331,7 +272,7 @@ namespace System.Globalization
 
         public override int GetDayOfMonth(DateTime time)
         {
-            return (GetDatePart(time.Ticks, DatePartDay));
+            return time.Day;
         }
 
         // Returns the day-of-week part of the specified DateTime. The returned value
@@ -351,7 +292,7 @@ namespace System.Globalization
 
         public override int GetDayOfYear(DateTime time)
         {
-            return (GetDatePart(time.Ticks, DatePartDayOfYear));
+            return time.DayOfYear;
         }
 
         // Returns the number of days in the month given by the year and
@@ -422,7 +363,7 @@ namespace System.Globalization
 
         public override int GetMonth(DateTime time)
         {
-            return (GetDatePart(time.Ticks, DatePartMonth));
+            return time.Month;
         }
 
         // Returns the number of months in the specified year and era.
@@ -452,7 +393,7 @@ namespace System.Globalization
 
         public override int GetYear(DateTime time)
         {
-            return (GetDatePart(time.Ticks, DatePartYear));
+            return time.Year;
         }
 
         // Checks whether a given day in the specified era is a leap day. This method returns true if
diff --git a/src/mscorlib/src/System/TimeZoneInfo.TransitionTime.cs b/src/mscorlib/src/System/TimeZoneInfo.TransitionTime.cs
index 0fe3dab0b6..90304c92ea 100644
--- a/src/mscorlib/src/System/TimeZoneInfo.TransitionTime.cs
+++ b/src/mscorlib/src/System/TimeZoneInfo.TransitionTime.cs
@@ -103,7 +103,8 @@ namespace System
                 }
                 Contract.EndContractBlock();
 
-                if (timeOfDay.Year != 1 || timeOfDay.Month != 1 || timeOfDay.Day != 1 || (timeOfDay.Ticks % TimeSpan.TicksPerMillisecond != 0))
+                timeOfDay.GetDatePart(out int timeOfDayYear, out int timeOfDayMonth, out int timeOfDayDay);
+                if (timeOfDayYear != 1 || timeOfDayMonth != 1 || timeOfDayDay != 1 || (timeOfDay.Ticks % TimeSpan.TicksPerMillisecond != 0))
                 {
                     throw new ArgumentException(SR.Argument_DateTimeHasTicks, nameof(timeOfDay));
                 }